Rather than food or water, the Mal’akh needed to eat the blood and flesh of the living in order to survive. Their own blood appeared like black bile. Furthermore, the Mal’akh were capable of changing their shape at will. According to the Liber Sanguisugarum, there were two types of Mal'akh, categorized by diet.

If a Mal'akh wanted to appear fair to human eyes, it needed to consume untainted and stable biomass (that is to say, human blood and flesh). The resultant rephaim were unnaturally beautiful, angelic creatures that could pass as humans (albeit exceptionally charismatic and magnetic ones). In the 18th century, they used this power to infiltrate the courts of the Turkish empire and Ali Pascha; the Ottoman army of Nicolopolis was known to be filled with Mal’akh in the 15th century. In legends of the Arabian and Middle Eastern peoples, they appeared as the Djinn and Peri, the spirits of the air and masters of desert storms.

The nephilim (metaphorical “giants of the Earth” mentioned in the Book of Genesis) were Mal'akh who, having stopped feeding on the blood of humans, turned upon themselves instead. Without the ingestion of a suitable, untainted biomass, the memetic field of the Mal'akh would be stretched too far, distorting their body and mind in unpredictable, animalistic ways. The resultant ape-like creatures were commonly known as grotesques or sometimes Babewyn. Their hideous shapes inspired many of the demonic images of Middle Eastern legend.

Richard Francis Burton encountered grotesques in his journey to the Mountains of the Moon, where he said that the more civilised Mal'akh would keep them as pets or force the change upon an enemy. His private accounts reference his companion's ravings about "crowds of devils, giants, lion-headed demons who avert wrenching with superhuman force..."

The legendary origins of the Mal’akh were recorded in the Biblicalapocrypha, namely the Book of Noah and Book of Enoch, where the Anakim (“watchers”) are a highly advanced and civilised race with many members who would prefer to be involved in worldly affairs instead of simply watching. Led by Azazel (or Shemjaza in some versions), two hundred rebels descended from “the high place” to live among the people, spreading their knowledge and taking wives. However, their offspring, created by the intermarriage of humans and the spirit world, were unstoppable monsters with a terrible hunger for flesh and blood. The watchers fought and captured the rebels, and bound their leader headfirst over the abyss for all eternity, hanging by one leg.

As the Mal’akh’s alter-time structures suggested a connection to the Great Houses, many commentators speculated that Enoch tells a corrupted account of the war against the Yssgaroth and the flight from the Homeworld of those tainted by Yssgaroth biomass. Faction Paradox always claimed that their armour was created from the bones of such agents from an alternate timeline where the Houses lost their war, though Richard Francis Burton, the leading scholar on the Mal’akh, disputed this. He claimed that the Yssgaroth/Great House hybrids were actually an inevitable result of the fighting of the war: even without the Homeworld being overrun, the Great Houses’ biomass was infected with the Yssgaroth taint, though they refused to acknowledge it. Burton went on to speculate that, given the similarities between the Houses’ regeneration abilities and those of the Mal’akh, the ancient Houses deliberately infected themselves to give themselves a biological advantage. Their paranoid reaction to the Faction’s armor was not a fear of paradox but a fear of the truth.